The invention concerns a load-responsive fluid pressure operable control unit for pneumatic trailer brake systems.
There are presently known control units whose structure is such that the load-control function of the brake pressure occurs after operation of the relay valve. This system complies with requirements of certain regulatory standards with respect to response and pressure build-up time, and also with respect to emergency braking in the event of separation or rupture of the trailer brake pressure line.
Regulations which define the relation of braking ratio static friction coefficient and require an installation of automatic load-controlled brake pressure controls on commercial vehicles, are satisfied by some presently known brake pressure control units with respect to the friction characteristic.
The mentioned brake pressure control unit represents a device, which practically also fulfills the function of a trailer brake valve in addition to the described functions. However, the required structural measurements effect the dimensions of the device so as to necessitate a large housing. Attaining brief venting periods requires large cross-sectional dimensions for the combined control relay valve, specifically for the venting process. Large cross sections again require large structural dimensions, which again demands greater operating forces at the tappet and therefore a heavier structure of the components due to the considerable pressure increase. Since relatively high clearance results when this relatively advantageous valve system is used as a trailer brake valve, the emergency control system which is activated upon separation or rupturing of the trailer brake pressure line, is practically located on the valve system.